The present invention relates generally to electromagnetic inspection tools, and more specifically relates to tools employing eddy-current techniques for determining the wallthickness of ferromagnetic casing.
Tools for electromagnetically determining the wallthickness of casings are known in the prior art. These tools commonly use eddy-current or flux leakage principles.
Eddy-current Electromagnetic Thickness Tools (ETT) for measuring casing thickness use a transmitter coil spaced from a receiver coil for a measurement within the casing for applications where it is impossible to gain access to the outside of the casing, e.g., the casing of an oil or gas well. The transmitter coil is energized at a frequency and is spaced from a receiver coil such that flux which is generated is detected by the receiver coil after the flux has passed twice through the wall of the casing.
As the magnetic flux passes through the casing wall, circular eddy currents are generated in the casing. The circulating eddy currents cause the phase of the transmitted flux to change by one radian each time the flux passes through a so-called skin depth. By measuring the magnitude of the phase change between the transmitted signal and the received signal, an approximation to the thickness of the casing layer can be determined as a function of the conductivity and permeability of the casing and the frequency of the signal which energizes the transmitter coil. Typically, however, recordings as a function of depth of the well or "logs" of the phase change measurements are made and compared with a base or reference log previously made for a casing. Comparison of the base log with the logs created during measurements made on the casing under inspection allows a general determination of change of thickness of the casing.
As indicated above, one of the primary applications for ETT tools is in determining the wall-thicknesses of pipe casings. As oil becomes increasingly valuable, more accurate and precise determinations of the wall-thickness of oil field casings are required. For example, it is very important to be able to determine when pits and other imperfections reduce the wall-thickness to a dangerously thin value which could either allow contaminants into the casing or oil to leak from the casing. The latter concern is especially important from both ecological and economic standpoints in not polluting underground environments. For example, oil leakage into fresh water aquifiers in desert areas may have a devastating impact.
It is recognized that the measurements made by existing ETT tools are sensitive to magnetic anomalies in the casing being measured. Present day ETT tools also are dependent on conductive anomalies in the casing, but fluctuations in permeability are far more influential on the wall-thickness measurement than are fluctuations in the conductivity. That is, permeability variations are more likely to occur more frequently and over a greater range of values than are changes in conductivity. Accordingly, if an ETT tool is to achieve a reasonably precise determination of thickness, it must at least take into account the actual permeability value of the casing under inspection.
Prior art proposals for taking permeability into account in determining casing thickness appear either to be economically unattractive or technically insufficient. For example, there have been ETT tool proposals for measuring casing thickness using coils which produce flux at three different frequencies. The proposals have suggested that this data be compared or otherwise processed to produce a more precise determination of casing thickness. The use of digital computer techniques employing predictive formulas and iterative techniques for analyzing the data taken at three separate frequencies has been suggested.
One prior art proposal has suggested monitoring a permeability signal simultaneously with a thickness signal to recognize permeability effects on the thickness signal. These proposals have not culminated in a commercially satisfactory solution to the need for a more precise Electromagnetic Thickness Tool (ETT) for measuring the thicknesses of ferromagnetic casings.
An important shortcoming of prior ETT tools is the fact that the phase shift measurement, while proportional to casing thickness, is also dependent on casing permeability. As indicated above, a base log must have been run for a particular casing, and the ETT log for the casing under inspection had to be compared to the base log in order to obtain an indication of a change in casing thickness. This technique has not only been cumbersome, but has given values only generally representative of wall-thickness.